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Diphenylketene reacted with the 2-azabicyclo[2.2.1]hept-5-enes 5, 6 and 10 by attack on nitrogen to give the piperidones 7, 8 and the cyclopentenols 11 respectively. The same ketene reacted with the cyclohexa-1,3-dienes 16 and 17 to give significant amounts of the [4 + 2] addition products 20 and 21 respectively. The initially formed product from the cycloaddition of the enol ether 23 and diphenylketene is unstable, rearranging to produce the unsaturated ester 24.
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Maurya et al. (1992) studied this question.
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